1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a developer cartridge capable of indicating the amount of remaining developer, and a developing unit and an image forming apparatus having the developer cartridge.
2. Description of the Related Art
An image forming apparatus such as, e.g., an electrographic laser printer, generally uses a dry-type developer, such as a powder developer, to develop an image. The amount of remaining developer inside a developer cartridge is checked to control the developing system and also to alert the user when it is time to replace the cartridge.
The powder developer is generally held in the developer cartridge, and typically requires an agitation in the developer cartridge. As the developer powder is caused to be moved around in the developer cartridge, sometimes resulting in a large amount of developer being moved to a particular location within the developer cartridge, it becomes difficult to measure with reasonable accuracy the amount of remaining developer in a developer cartridge when the measurement is taken from limited locations.
A suggestion has been made to estimate the amount of the developer remaining, based on the amount of printing that had been performed. For example, a devoted control unit may be provided to calculates an area of an electrostatic latent image formed on a photosensitive medium to estimate the amount of developer required to develop the electrostatic latent image. That is, the control unit may estimate an amount of required developer to print out one sheet of printing medium, and may keep track of an estimate of the amount of developer that may have been used, and thus estimates the amount of developer that may be remaining in the developer cartridge.
However, since the above method is based on an indirect estimation, there often is a discrepancy between the estimate and the actual used amount. For example, the actual amount of remaining developer may generally turn out to be smaller than the estimated amount as some additional amount of developer may have been actually used due to, e.g., developer leakage, loss, or the like during printing operations. Such less than accurate estimate may result in images being printed with less density, or even failing to be printed.